This invention relates to an airless spray nozzle adapted to spray a coating material such as paint by means of only a liquid pressure without using compressed air, and more particularly to an airless spray nozzle suitable for spraying a coating material of high viscosity without generating any tailing.
Conventionally, various kinds of airless spray nozzles have been proposed for the purpose of preventing generation of tailing on both sides of an elongated spray pattern as much as possible. For example, an airless spray nozzle for spraying a coating material of low viscosity such as melamine resin paint without using compressed air is disclosed in U.S. Pat. No. 3,659,787 corresponding to Japanese Patent Publication No. 4799/1972 and U.S. Pat. No. 4,905,911 corresponding to Japanese Patent Application Laid-Open Publication No. 178867/1988. The airless spray nozzle disclosed is generally constructed in such a manner that a substantially hemi-spherical depression or cavity opens rearwardly and has a groove provided across the depression to form a lenticular orificetype injection hole and a through-hole arranged rearward of the depression and connected to a paint feed source.
The spray nozzle disclosed in U.S. Pat. No. 3,659,787 is formed with an annular step at the boundary between the hemi-spherical depression and through-hole which provide a flow passage for paint in cooperation with each other. The step thus arranged would function to generate turbulence in the flow of paint at the rear of the injection hole to equalize the pressure or velocity of paint at the rear of the injection hole or the flow velocity of paint, to thereby prevent generation of tailing.
The spray nozzle disclosed in U.S. Pat. No. 4,905,911 is designed to prevent generation of tailing even when a paint injection pressure or a pressure applied to paint is reduced as compared with that in the spray nozzle taught in U.S. Pat. No. 3,659,787. The nozzle of U.S. Pat. No. 4,905,911 is provided on a part of the inner peripheral surface thereof which defines the through-hole with a pair of straight guides or main slants arranged opposite to each other, through which the through-hole is partially connected to the depression in a manner to be smoothly contiguous thereto. Also, the remaining part of the boundary between the through-hole and the depression other than the above-described part smoothly contiguous to the main slants is provided with a pair of steps which are defined between the opposite main slants and through which the through-hole and depression are discontinuously connected to each other. The main slants are positioned so as to be aligned with both end sides of the injection hole. It would be considered that the arrangement of such main slants causes the flow of paint along the main slants into the depression (straight flow of paint) to collide with the flow of paint deflected by the steps (deflection flow of paint), to thereby produce significantly increased turbulence at the rear of the injection hole.
Recently, the demand for an airless spray nozzle which permits resin paint of viscosity as high as 10 poise to be sprayed without generating tailing has increased. Unfortunately, it was found that the conventional airless spray nozzle described above fails to prevent tailing when a paint injection pressure is limited to a low level. This is for the reason that the conventional airless spray nozzle is designed to prevent tailing in the injection of paint of viscosity as low as Ford Cup 4 corresponding to 3 poise or less such as melamine resin paint.